The present invention relates to optoelectronic devices such as light-emitting diodes.
Light emitting diodes or xe2x80x9cLEDsxe2x80x9d include thin layers of semiconductor material of two opposite conductivity types, referred to as p-type and n-type. The layers are disposed in a stack, one above the other, with one or more layers of n-type material in one part of the stack and one or more layers of p-type material at the other end of the stack. For example, the various layers may be deposited in sequence on a substrate to form a wafer. The wafer is then cut apart to form individual dies that constitute separate LEDs. The junction between the p-type and n-type material may include directly abutting p-type and n-type layers, or may include one or more intermediate layers which may be of any conductivity type or which may have no distinct conductivity type. In operation, electric current passing through the diode is carried principally by electrons in the n-type layers and by electron vacancies or xe2x80x9cholesxe2x80x9d in the p-type layers. The electrons and holes move in opposite directions toward the junction and recombine with one another at the junction. Energy released by electron-hole recombination is emitted as light. As used in this disclosure, the term xe2x80x9clightxe2x80x9d radiation includes infrared and ultraviolet wavelength ranges, as well as the visible range. The wavelength of the light depends on factors including the composition of the semiconductor materials and the structure of the junction.
Electrodes are typically connected to the n-type and p-type layers near the top and bottom of the stack. The materials in the electrodes are selected to provide low-resistance interfaces with the semiconductor materials. The electrodes, in turn, are provided with pads suitable for connection to wires or other conductors that carry current from external sources. The pad associated with each electrode may be a part of the electrode, having the same composition and thickness of the electrode, or may be a distinct structure that differs in thickness, composition, or both from the electrode itself.
Some LEDs have electrodes on the bottom surface of the bottom semiconductor layer. For example, the various layers may be deposited in sequence on an electrically conductive substrate, and the substrate may be left in place on the bottom surface to act as a bottom electrode. Thus, LED""s formed from AlInGaP typically use conductive GaAs substrates and may have an electrical connection to the substrate. However, LEDs formed from certain semiconductor materials normally use nonconductive substrates to promote proper formation of the semiconductor layers. The nonconductive substrate typically is left in place, so that an electrode cannot be provided on the bottom surface of the bottom layer. For example, gallium nitride-based materials such as GaN, AlGaN, InGaN and AlInGaN are used to form LEDs emitting light in various wavelength ranges including blue and ultraviolet. These materials typically are grown on insulating substrates such as sapphire or alumina.
LEDs incorporating an insulating substrate must include a bottom electrode at a location on the stack above the substrate but below the junction. Typically, the upper layer or layers of the stack are removed after formation of the stack in a region covering part of the area of each die, so as to provide an upwardly-facing lower electrode surface on a layer at or near the bottom of the stack in each die. This leaves a region referred to as a xe2x80x9cmesaxe2x80x9d projecting upwardly from the lower electrode surface and covering the remaining area of the die. The area of the die occupied by the lower electrode surface does not emit light. It is desirable to keep the horizontal extent of this inactive area as small as possible.
The top electrode typically is formed on the top surface of the stack, i.e., the top surface of the top semiconductor layer, and makes ohmic contact with this top layer. Typically, the layers in the stack above the junction are transparent, so that light emitted at the junction can pass out of the stack through the top surface. The top electrode is arranged so that it does not block all of the emitted light. For example, an opaque top electrode may cover only a small portion of the top surface of each die. However, the current passing from such an electrode will tend to flow downwardly through the stack so that the current passes predominantly through the area of the junction disposed beneath the electrode. This phenomenon, referred to as xe2x80x9ccurrent crowding,xe2x80x9d results in light emission concentrated in that area of the junction beneath the electrode, precisely where it will be most effectively blocked by the electrode. The amount of useful light reaching the outside of the die per unit of electrical current passing through the die, commonly stated as the external quantum efficiency of the die, is reduced by this phenomenon. Current crowding is a significant consideration with LEDs formed from materials having relatively high electrical resistivity, such as the gallium nitride-based materials.
To alleviate the current crowding problem, LEDs have been provided with transparent top electrodes, formed from thin layers of metals and metal compounds. A pad, which is typically opaque, occupies a small portion of the top surface. The transparent top electrode spreads the current in horizontal directions from the pad, so that current flow down through the stack is spread more evenly over the horizontal extent of the mesa. However, the top electrode normally must be quite thin in order to make it transparent and minimize the amount of light absorbed by the electrode. Therefore, the transparent electrode typically has significant resistance to current flow in the horizontal directions. There may still be significant current crowding in the area beneath the pad of the top electrode pad unit.
Other types of LED""s use relatively thick semiconductor fusion and current diffusion layers above the junction. For example, AlInGaP type LEDs typically have a structure as shown in FIG. 1. LED 10 includes an n-type GaAs substrate 12. An n-type AlGaInP lower cladding layer 14, an AlGaInP active layer 16, a p-type AlGaInP upper cladding layer 18, and a thick p-type GaP layer 20. The GaP layer 20 serves as a current diffusion layer. A p-type electrode pad 22 is formed on the p-type GaP layer, and an n-type electrode 24 is formed on the lower face of n-type GaAs substrate 12. Due to the relatively thick GaP layer, a transparent top electrode is normally not required to allow the current to diffuse to the active layer 16. Nonetheless, the p-type electrode pad tends to block a significant part of the light emitted from the active layer. This results in reduced light extraction from AlInGaP LEDs. Despite the current spreading effect of the thick GaP layer, current crowding beneath the electrode pad is also a problem with AlGaInP LEDs. There is a need to alleviate the problems of current crowding and reduced light extraction beneath the electrode pad.
The present invention addresses these needs.
One aspect of the invention provides a light-emitting diode that includes a stacked structure. The stacked structure incorporates a first region of a first conductivity type, a second region of a second conductivity type and a light-emitting p-n junction between these regions. An upper portion of the first-type region defines a generally horizontal first contact surface.
The light emitting diode according to this aspect of the invention preferably includes a pair of electrodes for applying voltage to the diode. A first electrode preferably includes a first pad in contact with the first contact surface. According to this aspect of the invention, a reflector is disposed beneath the first pad, and the reflector includes walls that are oblique with respect to the first contact surface.
According to another aspect of the invention, the reflector is cone-shaped. In another aspect of the invention, the reflector is contained within the first region of the first conductivity type. In still another aspect of the invention, the reflector contains dielectric material.